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Quercus Douglasii) Saplings1 Effects of Fire on Naturally Occurring Blue 1 Oak (Quercus douglasii) Saplings Tedmund J. Swiecki2 and Elizabeth Bernhardt2 Abstract We studied the survival and regrowth of naturally-occurring blue oak saplings burned in a September 1996 arson fire in Vacaville, California. The saplings (pre-fire height 33-353 cm) were burned in a rapid, low-moderate intensity fire. Of 67 blue oak saplings surveyed, 4 failed to resprout after the fire and 2 more died within the following 5 years (9 percent mortality overall); all mortality was among saplings less than 100 cm tall. Saplings which were completely topkilled (51/67) were significantly smaller on average than those which were only partially topkilled (12/67). Saplings taller than 201 cm or with a stem diameter greater than 5.6 cm at 30 cm height were only partially topkilled. Only 20 percent of the completely topkilled saplings had regained both their post-fire height and diameter by 5 years after the fire. Height growth of new shoots from completely topkilled saplings was highest in the first year after the fire, but declined in succeeding years. High levels of vole damage in resprouted topkilled saplings has adversely affected growth and shoot survival. Our observations indicate that fire negatively impacts small blue oak saplings and does not favor blue oak regeneration, as has been suggested by other authors. Introduction Fire plays an important role in the maintenance of numerous plant communities in California but little is known about the role of fire in the sustainable management of California oak woodlands. Several lines of evidence have suggested that fire is necessary to prevent areas with black oak (Q. kelloggii) and Oregon oak (Q. garryana) from being invaded by conifers (Fritzke 1997, Hastings and others 1997). Change in fire frequency has been suggested as a reason for the failure of oak regeneration in many areas (Pavlik and others 1991), but there is no direct evidence indicating that fire actually aids regeneration of blue oak (Quercus douglasii). In a previous study on blue oak sapling recruitment in 15 locations, we found that sapling recruitment was not dependent upon fire and that repeated fires reduced sapling populations (Swiecki and others 1997). Similarly, Roy and Vankat (1999) found no blue oak regeneration when they resurveyed plots in Sequoia National Park, some of which had burned. Mortality of mature trees was greater in burned plots, but the paper does not report whether the differences were statistically significant. Employing dendrochronlogical methods to age trees and fires, Mensing (1992) and McClaren and Bartolome (1989) showed an association between blue oak stem age 1 An abbreviated version of this paper was presented at the Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape, October 22-25, 2001, San Diego, California. 2 Principals, Phytosphere Research, 1027 Davis Street, Vacaville, CA 95687 (e-mail: [email protected]) USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 251 Effects of Fire on Blue Oak Saplings—Swiecki and Bernhardt and fire occurrence. Some authors (McClaren and Bartolome 1989, Mensing 1990, Pavlik and others 1991) have suggested that this association supports the hypothesis that fire promotes blue oak regeneration, perhaps by accelerating growth rates of post-fire shoots and suppressing competitive weedy vegetation. An alternative explanation of the historical association between fire and blue oak regeneration is that fire destroys the tops of young, pre-existing blue oaks which resprout after the fire. Under this hypothesis, cohorts of new sprout-origin trees would all date to the time of the fire occurrence. Because this recruitment is the direct result of fire-induced topkill of existing oaks, fire does not cause a net increase in the oak population and thus has no positive effect on regeneration. In this study, we observed the fate and regrowth of naturally-occurring blue oak saplings burned during an arson fire in Vacaville, California. The objectives of this study were to document the sensitivity of small blue oak saplings to topkill by fire and to determine the length of time required for new shoot growth to replace shoots destroyed by fire. Partial results of this study have been reported (Swiecki and Bernhardt 1999). Methods The study site is on City of Vacaville open space lands located south of Interstate 80 on the northeast end of Lagoon Valley. The hillside supports a mixed oak woodland dominated by blue oak, valley oak (Q. lobata), interior live oak (Q. wislizeni), and California buckeye (Aesculus californica). The woodland, some of which was previously cultivated as orchards, contains a number of large old clearings. Vegetation in the clearings consists mostly of nonnative annual grasses and weedy forbs, but we had observed relatively high numbers of small blue oak seedlings and saplings along the edges of clearings. The complete grazing history of the study site is not known. The area was not grazed during the study and has not been grazed at least for 10 years prior to the study. In late September 1996, an arson fire burned much of the open grassland areas and partway into the understory of the adjacent oak woodland. Fire fuels in the openings where the blue oak saplings were located consisted primarily of dry annual grasses and forbs, which were completely consumed in the rapid, low-moderate intensity fire. No mature trees were killed by the fire although a few mature interior live oak trees (Q. wislizeni) in the burned area developed moderate to severe trunk scarring after the fire. Due to characteristics of the fire, the small blue oak saplings at the site were heat-killed but not consumed by fire. We were therefore able to locate and measure the height and diameter of the heat-killed stems, which were still standing and intact at the time of our initial survey (June 2-5, 1997). At that time, we located and tagged 67 blue oaks saplings located in several groupings in the burned area. We recorded the post-fire status of each sapling (partially topkilled, completely topkilled with live basal sprouts, or dead). For completely topkilled saplings, we measured the height of the tallest heat- killed stem, as well as the diameter at ground level and at 30 cm (D30) of the largest stem. Due to dehydration of killed stems prior to initial measurements in June 1997, these pre-fire stem diameters are likely to be somewhat smaller than actual pre-fire diameters of live stems. We also counted the number of heat killed and live shoots 252 USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. Effects of Fire on Blue Oak Saplings—Swiecki and Bernhardt and the height of the tallest live shoot. Because these sprouts were very thin (generally 0.5 cm or less) we did not attempt to measure their diameters in the first post-fire survey. For partially topkilled saplings we recorded the height of the tallest branch prior to the fire. This was the greater of the tallest dead shoot or the height to the previous season's live shoot tip (based on the location of terminal bud scars). We measured the height of the tallest currently live stem, and stem diameters at ground level, 30 cm, and 137 cm diameter at breast height (DBH) if applicable. We also visually estimated the percent of the canopy which had been heat-killed. Tagged saplings were relocated and reevaluated annually through July 2001. Annual measurements were made in midsummer or later, after the current season's shoot was largely complete. Because basal diameter and D30 measurements were highly correlated and D30 measurements were much easier to make, we only measured D30 and DBH for the largest stem of each saplings after 1997. Only D30 stem diameters are reported in this paper. We also measured the height of the tallest shoot, the number of stems, and rated the presence and severity of meadow vole (Microtus californicus) damage in the annual evaluations. Data for one to two saplings are missing in certain years because they were not located during the survey. We used JMP statistical software (SAS Inc., Cary NC)3 for data analysis. Unless otherwise indicated, effects or differences are referred to as significant if P≤0.05. We used logistic regression models to examine the effects of plant factors on the binary outcome total or partial topkill. We used paired t tests to compare heights of saplings made in different years. Because delayed topkill and mortality occurring during the study caused annual changes in the number of trees in each status category, the sapling counts are presented along with percentages if based on a total other than all 67 tagged saplings. Results Effects of Fire on Plant and Shoot Survival Of the 67 saplings included in the study, 12 (18 percent) were only partially topkilled as of June 1997. In June 1997, levels of crown damage in partially topkilled saplings ranged from less than 2.5 percent to over 80 percent crown dieback. Two saplings that showed only partial topkill in June 1997 died back to root sprouts between June 1997 and October 1999; both of these saplings had more than 50 percent crown dieback in June 1997. These saplings are included in totals for completely topkilled saplings in the data for 1999 onward. The fire caused complete topkill of aboveground shoots in 55 (82 percent) of the saplings. Four topkilled oaks (6 percent) failed to resprout and were dead as of June 1997. In addition, 2 oaks that had resprouted from topkilled saplings died between the 1999 and 2001 evaluations. Hence, by 5 growing seasons after the fire, overall mortality was 9 percent (6/67) and 84 percent (51/61) of the surviving oaks were represented by resprouts whose shoots originated after the fire.
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